Crude oils and fractions thereof are typically processed first by fractionating in a refinery and then by cracking, such as in a pyrolysis furnace, to yield various products including the light olefins ethylene, propylene, and butylenes.
Conventional steam cracking utilizes a pyrolysis furnace which has two main sections: a convection section and a radiant section. The feedstock typically enters the convection section of the furnace where it is heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products, including the valuable light olefins mentioned above, leave the pyrolysis furnace for further downstream processing, including quenching and recovery from one or more fractionating columns/towers.
The crude oils and fractions thereof which contain asphaltenes and other heavy molecules having a high final boiling point (FBP) cannot be used directly as feed in conventional steam cracking processes because the asphaltenes will become fouling precursors in the pyrolysis furnace. The fouling is in the form of coke deposits and the like, which negatively affect the furnace performance due to increased pressure drop, reduced heat transfer, plugging, and other problems.
Materials such as condensates and naphthas are often contaminated by heavy molecules having a high FBP. For instance, condensates and naphthas are often transported in containers such as ships which have previously contained crude oil, heavy gas oil, resids, and the like, having FBP greater than 950° F. (510° C.) and small but significant amount of material having FBP of 1200° F. (650° C.) and higher. Condensates may also be obtained from the gasfields contaminated with these high FBP molecules. Once contaminated, these materials have decreased value as pyrolysis furnace feeds because they cause fouling. The location of the fouling is the key problem for subsequent decoking operations.
The present inventors have noted that the problem of fouling in a pyrolysis furnace is particularly acute with pyrolysis feed fractions containing light molecules in addition to the heavy foulant molecules. In this instance, the specific problem is that that foulant deposits (often referred to as “coke”) are formed in the tube banks located in the upper part of the convection section of the pyrolysis furnace which cannot be removed during decoking operations. The problem applies to furnaces with and without transfer line exchangers used to quench the furnace effluent (hereinafter “TLE furnace”).
With TLE furnaces, deposits may form above the HP (high pressure) steam superheater rows of the furnace convection section. During decoking operations, the temperature of the furnace (and thus the temperature of the air/steam decoking mixture) above the HP steam superheater rows is too cold to burn the coke deposits. The temperature in the upper convection section (and thus the temperature of the air/steam decoking mixture) is generally too low to facilitate decoking because of the energy used in heating the saturated steam in the HP steam superheating tubes located between the upper and lower convections section tube banks. As a result, coke that deposits in the tubes in the lower temperature regions of the convection section can only be removed by first shutting down and cooling the furnace, and then hydroblasting to remove the coke. This mechanical cleaning is expensive, not the least of the expense due to lost production time.
In the past, the above mentioned problems could be addressed by redistilling the feed material to obtain clean feed, but this is an energy intensive and wasteful solution. Diluting the contaminated feed with clean feed is a known procedure, but this does not solve the problem of deposits laying down in tube bank locations that are inaccessible to ordinary decoking operations. More generally, contamination problems in refinery and/or pyrolysis feeds have been addressed by various methods such as membrane separation (U.S. Pat. No. 6,013,852); addition of materials such as an oil soluble overbased magnesium sulphonate (U.S. Pat. No. 4,931,164), a “free-radical acceptor”, e.g., n-heptane (U.S. Pat. No. 3,707,459), toluene (DD 222324), coal-derived gasoline (DD 218116), or deisobutanized C3 to C5 paraffin stream (U.S. Pat. No. 3,922,216); recycling of a portion of the product, such as naphtha from the cracked product stream (U.S. Pat. No. 4,178,228); and pretreatments such as hydrogenation of heavier fractions (GB 2006259). These methods suffer, among other reasons, by not being generally applicable to feeds comprising crude or crude fractions, not solving the problem of coking on modern equipment, and/or being inefficient.
U.S. Patent Application 2005/0261535 describes a method of processing light hydrocarbon feedstock containing non-volatile components and/or coke precursors comprising flashing the feed in a flash/separation vessel (whereby asphaltenes are removed in the liquid phase) and cracking the asphaltene-free vapor phase of said flash/separation vessel.
The present inventors have surprisingly discovered that mixing heavy feed with contaminated light feed enables decoking of the foulant and eliminates the need for costly mechanical decoking of a pyrolysis furnace.